This invention relates generally to process control, and, more specifically, to the processing of a layer through a surface thereof and the detection of an endpoint to the process by optical techniques.
There are many situations where such processing takes place. One class of such situations occurs in the manufacturing of flat panel liquid crystal displays and integrated circuits on semiconductor wafers. In each, several processing steps include forming a layer of material and subsequently removing either all of the layer or a portion of it according to a pattern. As part of this process, it must be accurately determined when just enough of the layer has been removed; i.e., to detect an endpoint of the removal operation. The endpoint determination is then used to monitor the progress of the process and/or to control the process, such as by automatically terminating the specific processing operation being monitored.
At several stages in the manufacture of display panels or circuits on a semiconductor wafer, a mask is formed of photoresist material. The resulting mask is used to limit processing of a layer covered by the mask to a patterned area. The mask is formed by exposing the photoresist layer to light in the desired pattern, followed by developing the photoresist layer through application of a developer solution to it. With the usual photoresist material, the exposed regions are removed during the development process to expose the layer below. The time at which the underlying layer first becomes exposed by removal of photoresist material is termed the "breakthrough" or "endpoint." The development process is allowed to continue for a period of time after breakthrough is first detected, the end of that period of time being the end of the development process, termed its "process end" (or "step end" if there is more than one step in the overall process).
Because of various processing and environmental variations that exist among semiconductor wafers and flat panel display substrates, the development process is monitored in order to determine when breakthrough occurs. A beam of light having a finite bandwidth is directed against the photoresist layer from the side of the wafer or other substrate that carries the layer. A light signal reflected from the layer is then usually detected, although light transmitted through the structure is sometimes used. A resulting electrical signal is then processed to determine when breakthrough occurs.
In a common arrangement, light reflected from both the top and bottom surfaces of the substantially transparent photoresist layer interfere at, and is detected by, a photodetector. As a portion of the photoresist layer is removed during the development process, the detected intensity of the reflected light cycles between one or more maxima and minima as the material removal alters the relative phase between the two interfering beams. At breakthrough, however, this signal variation ends, a condition which is detected by analyzing the photodetector output signal. Development is then usually allowed to proceed for a fixed or calculated time after detection of breakthrough, at which point the development is terminated by rinsing away the development solution or by some other means.
Wet etching processes, wherein substantially transparent material layers other than photoresist material are etched away, also use such a breakthrough detection process. Where the layer being etched is opaque, however, the photodetector signal remains essentially level until breakthrough occurs, at which time the optical signal either suddenly rises or falls to indicate that breakthrough has occurred. These breakthrough detection techniques are used when either an entire layer is being etched away or when only a portion according to a pattern designated by a mask is removed by etching through openings of a mask that is resistant to the etchant.
In both of the photoresist development and wet etching processes, unless the article is totally immersed in the development or etching solution, such a solution is applied to the exposed layer by spraying, pouring or the like. This can interfere with the optical signal path used for endpoint monitoring and thus give inaccurate results, if endpoint can be determined at all. A mist or fog is often created in the space adjacent the layer being processed, particularly when the development or etching solution is applied by spraying. This has the effect of blocking or scattering the endpoint detecting beam, an effect which can change over time. If the processing occurs in a chamber having transparent walls or a viewing window, vapor of the solution can condense on an inside surface in a manner to attenuate any light beam passing through it. Since the wafer or other substrate is usually positioned horizontally, liquid can also puddle on the exposed surface of the layer being treated, thus affecting the light beam passing through. Liquid puddling can occur as a result of spraying or pouring the solution onto the layer surface. As an additional complication, these light beam attenuation and scattering effects can change over time.
Another difficult situation for optical endpoint detection exists during mechanical removal of material. A mechanical removal step is not uncommonly performed during both integrated circuit and flat panel display processing, for example. One specific application is to form a planar surface on a layer that has been deposited over a very uneven surface resulting from prior processing steps. An uneven exposed surface of the deposited layer is then planarized. Other times, it is desired to form a layer of substantially uniform thickness on a smooth underlying surface. In either case, the layer of material is initially formed with a thickness much greater than that desired, and part of it is removed by a mechanically abrasive process. In another specific application, the layer is either entirely removed, if the underlying surface is smooth, or removed in only its thinnest regions when the underlying surface is quite irregular. Accurate detection of the endpoint of these processes is, of course, desired.
Another mechanical removal process that is often performed is polishing. Microscopic variations of a surface are removed by polishing the surface. It is desired to be able to accurately detect when the surface roughness has been removed in order to avoid removal of more material than necessary. Polishing is performed on bare substrates and on layers of material formed on the substrates.
Both of the planarization and polishing processes are typically performed by holding the substrate up-side-down by means of a vacuum chuck or the like, and then rotating the chuck and substrate while the surface being removed is held in an abrasive fluid against a moving block. The size of particles in the abrasive fluid, the force with which the surface is urged against the block and the speed of rotation are adjusted for the specific planarization or polishing operation. Access from this surface, in order to optically monitor its progress with known techniques, is not possible during such an operation.
Therefore, it is a general object of the present invention to provide optical techniques for monitoring processing endpoints under those conditions described above and others presenting similar difficulties.
It is another object of the present invention to provide improved techniques for monitoring microscopic roughness and other variations in a surface.
It is another object of the present invention to provide an improved method of manufacturing large, flat liquid crystal display panels with an improved yield.